To avoid overrunning the internal buffer we need to take the offset into
the buffer into account.

Fixes: d95da91fb497 ("x86/HVM: grow MMIO cache data size to 64 bytes")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v2: New.

--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -935,7 +935,7 @@ static int hvmemul_phys_mmio_access(
     }
 
     /* Accesses must not overflow the cache's buffer. */
-    if ( size > sizeof(cache->buffer) )
+    if ( offset + size > sizeof(cache->buffer) )
     {
         ASSERT_UNREACHABLE();
         return X86EMUL_UNHANDLEABLE;